Concrete Calculator

1. Choose imperial (feet and inches) or metric (meters and centimeters).
2. Enter the slab length, width and thickness.
3. Read the volume in cubic yards and cubic meters, plus an estimated bag count, updating as you type.

Here is what each figure is for:

Concrete is sold by volume, and in the United States that volume is the cubic yard: a cube one yard on every side, equal to 27 cubic feet or about 0.765 cubic meters. A single cubic yard covers roughly 81 square feet at four inches thick, which is why slabs are quoted in yards rather than bags once they pass a certain size.

Everywhere else, concrete is ordered in cubic meters. The two units describe the same material, so this calculator shows both, letting you talk to a US ready-mix plant and an international supplier without redoing the math.

The tool multiplies the three dimensions to get a volume, converting the thickness so every measurement shares the same unit before it multiplies.

In imperial mode it divides the thickness in inches by 12 to reach feet, multiplies the three sides for cubic feet, then divides by 27 for cubic yards. In metric mode it divides the thickness in centimeters by 100 to reach meters and multiplies straight to cubic meters.

1. Put all three dimensions into the same unit.
2. Multiply length by width by thickness for the raw volume.
3. Convert to cubic yards or cubic meters, then divide by the bag yield for a bag count.

Bag counts assume the typical yields of 0.6 cubic feet for an 80-pound bag and 0.45 for a 60-pound bag.

Take a 10 by 10 foot slab, 4 inches thick. The thickness is 4 divided by 12, or 0.333 feet, so the volume is 10 times 10 times 0.333, which is about 33.3 cubic feet.

Dividing 33.3 by 27 gives roughly 1.23 cubic yards, or about 0.94 cubic meters. At 0.6 cubic feet per 80-pound bag that is about 56 bags, which is exactly the point where a ready-mix delivery starts to look easier and cheaper than hauling bags.

Drop the thickness to a 3-inch path and the same footprint needs only about 0.92 cubic yards, showing how strongly thickness drives the total.

Bagged concrete comes in a few standard weights, and their yields decide how many you carry.

Because a cubic yard is 27 cubic feet, dividing 27 by the bag yield gives the bags per yard. Once you are near a full yard, the bag count and the effort both climb quickly.

The footprint is only part of the story; a few factors move the real quantity up or down.

Thickness

Volume scales directly with depth, so going from 4 to 6 inches adds fifty percent, often the single biggest swing in the order.

Waste and Spillage

Some concrete sticks to the mixer, spills, or fills low spots, so a 5 to 10 percent allowance keeps you from coming up short.

Subgrade Evenness

An uneven or over-excavated base quietly swallows extra concrete beyond the nominal thickness you planned.

Shape and Edges

Thickened edges, footings and curves add volume that a plain length-by-width figure misses.

Small Repairs and Posts

For a few cubic feet, bagged concrete you mix on site is cheaper and avoids a delivery minimum.

Slabs Near or Above a Cubic Yard

Once the volume reaches roughly one cubic yard, ready-mix is usually cheaper per unit and far less labor than dozens of bags.

Budgeting the Pour

Knowing the cubic yards up front lets you price both options and compare them against the delivery cost before you commit.

1. Mixing Units

Keep length, width and thickness consistent. This tool converts the thickness for you, but only if you pick the right mode.

2. Forgetting a Waste Allowance

Order the calculated volume plus about 10 percent so a small overrun does not stop the pour.

3. Ignoring Thickened Edges

Footings and edge beams hold more concrete than the flat slab figure suggests.

4. Rounding Bags Down

Always round the bag count up; running one bag short mid-pour ruins the finish.

5. Treating Bag Yield as Exact

Yields vary a little by product, so read the bag and adjust if it differs from the standard figures.

The volume math is exact for a rectangular slab, and the conversions use standard factors, but real pours carry variables the geometry cannot see.

Multiply length by width by thickness, keeping all three in the same unit. This calculator does that and converts the result into cubic yards, cubic meters and an estimated bag count.

There are 27 cubic feet in a cubic yard, because a cubic yard is 3 by 3 by 3 feet. Divide cubic feet by 27 to get cubic yards.

About 45 of the 80-pound bags, 60 of the 60-pound bags, or 90 of the 40-pound bags, based on their standard yields.

An 80-pound bag yields roughly 0.6 cubic feet of mixed concrete, enough to cover about 1.8 square feet at four inches thick.

For small jobs under about a cubic yard, bags are cheaper and simpler. At or above a cubic yard, ready-mix is usually cheaper per unit and much less labor.

Add about 5 to 10 percent for spillage, an uneven subgrade and thickened edges, so a small overrun never halts the pour.

Four inches suits patios and walkways, while driveways and load-bearing slabs often need six inches or more. Follow local codes and any engineer guidance.

Yes. Switch to metric to enter length and width in meters and thickness in centimeters, and read the result in cubic meters.

No. The calculation runs in your browser, and nothing you type is sent anywhere unless you Save a result, which stays on this device only.

• Cubic yard (Wikipedia).
• Concrete (Wikipedia).
• Volume of shapes (Maths Is Fun).
• US customary to metric conversions (NIST).